In a permanent magnet electric machine the magnets are often secured to the rotor in a planar array by an adhesive. Positioning of the magnets on the rotor is a difficult and time consuming exercise because the magnets tend to be attracted to each other or repel each other, depending upon the orientation of the magnets. It is therefore important that the magnets be precisely placed in a position in which the magnetic forces are balanced. For example, the magnets may be oriented at an equal distance from one another in an alternating pole configuration, i.e. North, South, North, South etc. This arrangement would produce an attractive force between the magnets, which results in a net force of zero on each magnet if the equal distance between magnets is carefully maintained in a precarious equilibrium.
Given this issue, it is also important that the magnets be held in that position at least until the adhesive has properly cured. The curing time of the adhesive thus makes handling of partially assembled rotors a delicate exercise because any movement of the magnets outside of their ideal positions will cause them to move further, until they eventually come together. It is therefore undesirable to handle partially assembled rotors before the adhesive is completely cured. This slows down the production process and/or requires a large amount of valuable work space to be dedicated to the storage of rotors whilst the adhesive cures.
In a rotating motor having magnets attached to a rotor, centrifugal forces on the magnets during operation of the motor tend to pull the magnets radially outward and away from the rotor. If the sole means by which the magnets are secured to the rotor is an adhesive, the speed of the motor is thus limited by the strength of the adhesive. In this type of construction it is thus essential that the operating speed of the motor be kept well below a speed at which the centrifugal forces generated upon the magnets would overcome the adhesive bond holding the magnets to the rotor. Failure of the adhesive bonds could cause the magnets to be dislodged and subsequently damage the motor.
Magnets used in electric motors may be coated or plated to prevent corrosion. Thus, any adhesive which is used to secure a magnet to a rotor is in fact merely securing the anticorrosion coating or plating to the rotor. The strength of the bond between the actual magnet and the rotor is thus limited to the strength of the bond between the anticorrosion coating/plating and the magnet material. The use of adhesive alone is therefore not ideal for securing magnets to a rotor but it is still the norm.
With the foregoing difficulties in assembly and operation of permanent magnet electrical machines in mind, there is a need for a device which could facilitate the positioning of magnets within a permanent magnet electrical machine, such as a motor. It would also be desirable, although not essential, for the device to be able to remain within the assembled machine to assist in retaining the magnets to the rotor in their correct positions, with or without the use of adhesive, during operation of the motor.